Conventional optical fiber amplifiers include active fibers having a core doped with a rare earth element. Pump power at a characteristic wavelength for the rare earth element, when injected into the active fiber, excites the ions of the rare earth element, leading to gain in the core for an information signal propagating along the fiber.
Rare earth elements used for doping typically include Erbium (Er), Neodymium (Nd), Ytterbium (Yb), Samarium (Sm), and Praseodymium (Pr). The particular rare earth element used is determined in accordance with the wavelength of the input signal light and the wavelength of the pump light. For example, Er ions would be used for input signal light having a wavelength of 1.55.mu.m and for pump power having a wavelength of 1.48.mu.m or 0.98.mu.m; codoping with Er and Yb ions, further, allows different and broader pump wavelength bands to be used.
Traditional pump sources include single mode laser diodes and multimode broad area lasers coupled to the active fiber over single mode and multimode pumping fibers, respectively, to provide the pump power. Single mode lasers provide low pump power, typically in the order of 100 mW. Broad area lasers, on the other hand, provide high pump power, in the order of 500 mW. These lasers of high output power, however, cannot efficiently inject light into the small core of a single mode fiber. Consequently, the use of high power broad area lasers requires the use of wide core and multimode fibers for pumping optical amplifiers.
Broad area lasers generate multimode pump power and input the pump power to a non-active pumping fiber. This non-active pumping fiber in turn typically inputs the pump power through a coupler and into the inner cladding of a double-clad active fiber, acting as a multimode core for the pump power.
In the amplifier fibers, pump power is guided into the inner multimode cladding of the fiber from which it is transferred into a single mode core doped with an active dopant.
A fused fiber multimode coupler has a theoretical coupling coefficient directly proportional to the ratio of the areas of the two fibers constituting the coupler itself. In an ideal case for two identical fibers, the coupling coefficient is approximately 50%. Typically, the coupling coefficient is in the range of 45-48%. This means that only about 45-48% of the total pump power output by the pump source into the pumping fiber actually passes from the pumping fiber to the inner cladding of the double-clad active fiber, while the remaining 52-55% remains in the pumping fiber.
Some systems use two optical fibers having different diameter of multimode cores to improve the coupling coefficient of the multimode coupler. However, such arrangements often lead to a waste of power due to the difficulty in matching the tapering of two cores of different size.
FIG. 1 is a block diagram of a conventional amplifying system containing a multimode pump source coupled to a primary fiber via a single traditional coupler. Primary fiber 1100 is a double-clad fiber. The information signal flows through its single mode core. Amplifier 1200, which may take the form of an Er/Yb doped double-clad active fiber, amplifies the information signal as it propagates through the single mode core of primary fiber 1100.
Multimode pump 1300 is coupled into primary fiber 1100 via multimode pump fiber 1400 and coupler 1500.
Multimode pump power generated by multimode pump 1300 is coupled into primary fiber 1100 via multimode pump fiber 1400 and coupler 1500. Coupler 1500 is a conventional fused fiber wavelength division multiplexer (WDM) type coupler. WDN couplers behave as multimode couplers for the pump power and transmit the single mode signal along the primary fiber substantially without coupling to the pump fiber. WDM couplers have maximum coupling efficiencies of 50% for the pump power, and typically have coupling efficiencies in the range of about 45%.
Multimode pump 1300 may take the form of a broad area laser that outputs pump power of approximately 450-500 mW. Due to the coupling coefficient of coupler 1500, however, only about 45% of this pump power, or approximately 200-225 mW, enters primary fiber 1100. The remaining 55% of the pump power is lost, as it exits from pump fiber 1400.
More recent systems have attempted to recover the lost pump power. FIG. 2 is a block diagram of one of these systems. The primary fiber through which the information signal flows includes signal fiber 2100 and signal fiber 2200, which are matched double-clad fibers, and amplifiers 2300 and 2400. Amplifiers 2300 and 2400, which may comprise Er/Yb doped double-clad active fibers, amplify the information signal as it propagates through the single mode core of the primary fiber.
In this system, multimode pump 2500 is coupled into the primary fiber via two identical couplers 2600 and 2700, over pump fiber 2800 and pump fiber 2900, respectively. Pump fibers 2800 and 2900 are matched multimode fibers, spliced together at a point between couplers 2600 and 2700.
Multimode pump 2500 outputs multimode pump power over pump fiber 2800. Due to the coupling coefficient of coupler 2600, only about 45% of the pump power enters signal fiber 2100. With this arrangement, however, the remaining 55% of the pump power is not lost, but, instead, enters pump fiber 2900, which couples into signal fiber 2200 via coupler 2700.
Applicants have observed that the addition of pump fiber 2900 and coupler 2700, however, does not significantly improve the total pump power transfer over the one coupler system described above. There are a few reasons for this. First, the splice between pump fiber 2800 and pump fiber 2900 results in a loss of pump power. Second, the first coupling between pump fiber 2800 and signal fiber 2100 results in most of the external modes of the pump power being transferred to signal fiber 2100, leaving only the internal modes for the second coupling. This leads to inefficient transfer of the remaining pump power at coupler 2700. Such a structure attempts to recouple most of the internal modes of the multimode pump power, those with the poorest coupling efficiency. As a result, the coupled pump power and the multimode amplifier's output power do not change significantly over the single coupler system described above.
Several articles in the patent and non-patent literature address multimode coupling techniques but do not overcome the deficiencies of other conventional approaches described above. WO 95/10868 discloses a fiber optic amplifier comprising a fiber with two concentric cores. Pump power provided by multimode sources couples transversely to the outer core of the fiber through multimode fibers and multimode optical couplers. The pump power propagates through the outer core and interacts with the inner core to pump active material contained in the inner core.
U.S. Pat. No. 5,185,814 discloses an optical communications network in which amplifiers amplify optical signals as the optical signals propagate along a waveguide. A single optical pump source coupled to the optical fiber pumps the amplifiers.
U.S. Pat. No. 5,533,163 discloses a double-clad optical fiber configuration that includes a core doped with an active gain material, and-an inner cladding surrounding the core. An external pump source inputs multimode pump energy to the inner cladding. The multimode pump energy in the inner cladding transfers energy into the core through repeated interactions between the energy and the active dopant within the core.
WO 93/15536 discloses a compound glass fiber that includes an outer cladding, inner cladding, and a single mode central core. The inner cladding has a cross-sectional profile optimized to receive multimode pumping radiation. The single mode core is located within the inner cladding and doped with a lasant material to maximize transfer of the multimode pumping radiation to the single mode doped core.
U.S. Pat. No. 5,170,458 discloses an optical fiber light-amplifier that includes an optical fiber with a center core through which signal light propagates, and a pumping light generating device that applies pumping light obliquely to the optical fiber. While the pumping light propagates through the fiber, it is absorbed by and excites the center core to amplify the signal light propagated through the optical fiber.
In Applicants' view, none of the known literature has adequately addressed Applicants' discovery that conventional systems have failed to recouple sufficient pump power, thereby leading to an inadequate overall pump power transfer efficiency.